VACUUM. 



VACUUM. 





certain that the awakening of the attention of philosophers to the true 

 condition of the subject involved, datce from about that time.* This 

 mult i* of course fatal to the admismon of a tnie physical vacuum. It 

 to remarkable that thia result had been clearly anticipated by Newton 

 himself . who held that the planetary apace* could not really be vacuoua 

 because they were traversed by the force of gravity. Thia remarkable 

 expression of Newton's sagacity and power of thought has hid a iiingular 

 fate. Recorded in 1 898, it remained unpublished for nearly a century, 

 when It appeared (in 1783) in Horslcy's edition of his work*. Hut it 

 seem* to have remained unregarded until it was brought forward by 

 the celebrated Profwwor Playfair, o Ute as the year 1819 more than 

 a century and a quarter after its original enunciation and that merely 

 for the purpose of proving " that Newton did not consider gravity aa a 

 property inherent in matter." But even Playfair did not perceive the 

 entire force of the passage, and, indeed, the light which electricity w.i.- 

 deatined to throw on all branches of molecular physics had not then 

 been received ; Faraday had not yet replied to Playfair's quint ion, " it is 

 not quite clear in what manner the interposition of a material substance 

 can convey the action of distant bodiea to one another." t A third 

 part more of a century was destined to elapse, when Faraday cited 

 Newton's expression, to the general surprise of the scientific world, in 

 one of bin discourses at the Royal Institution, in 185S, in order to show 

 that Newton " was an unhesitating believer in physical lines of gravi- 

 tating force," and must be ranked "amongst tho-e who sustain the 

 physical nature uf the lines of magnetic and electrical force." J The 

 irreconcilability of the conception of lines of physical force with that 

 of an absolute vacuum, identities the subject now reviewed with that 

 of the present article. 



We proceed to notice some modern and comparatively recent expe- 

 rimental investigations, involving the production of nearer ami nearer 

 approximations to a physical v.icuum, or the more and more complete 

 removal of ponderable matter from an inclosed space. It is matter of 

 this kind only, the quantity of which, in a given space, can at all be 

 diminished ; th.it of the matter of a higher order, the ether which 

 manifests and transmits heat and light and perhaps magnetism, accord- 

 ing to our present experimental means, cannot be affected ; though 

 apparently acted upon in a certain munnner by the molecules of 

 ordinary matter, it cannot be confined or diminished in amount, any 

 more than it can be measured or weighed. Or, to express this in 

 terms independent of theory, a vacuum transmits light and heat, 

 diminished only by the imperfect transparency and transcolescence of 

 the including vessel. In the investigation in question, more or less 

 perfect vacua have been obtained by the air-pump ; others on the prin- 

 ciple of the space void of air left above the mercury in the barometer, 

 called the Torricellian vacuum ; some by the combination of both these 

 means ; and others again by the union of one or both with chemical 

 agency, by which apparently the most perfect vacua have been produced. 



Dr. Thomas Andrews, F.R.S., Vice-President of Queen's College, 

 Belfast, whose refined physico-chemical researches have required the 

 use of the nearest approach to a perfect vacuum in which certain 

 instruments could be observed, has devised a method of obtaining pro- 

 bably a more perfect air-pump vacuum than hod before been produced. 

 He characterises the Torricellian as the nearest approach to a perfect 

 vacuum which at the time when his method was devised had been 

 obtained " It is true," he remarks, " that it contains a little mer- ' 

 curial vapour at the ordinary temperature of our summers, and probably j 

 also at lower temperatures, but the quantity is exceedingly small, and 

 its influence in depressing the barometric column must be altogether 

 inappreciable. Besides the mercurial vapour, a trace of air may gene- 

 rally be detected." l)r. Andrews shows that it is easy to calculate 

 approximative^ the depression of the column produced by this ; and 

 he finds that if the diameter of the bubble be O'u2 inch, the pressure of 

 mercury under which it has been measured 2 inches, and the volume 

 of the space above the mercury when the tube i .: cubic 



inch, the depression of the mercurial column is nearly O'OUOul ; " or 

 the depression of xhe mercury, in consequence of the vacuum not being 

 absolutely perfect (with respect to air), amounts only to jj of an 

 inch. It is easy in actual practice," Dr. Andrews continues, " to realise 



fee a critical notice of FinulaT'i Exp. He..' in Phil. Mag.,' June, mo 



MT. ill , TOl |Y., p. 400. 



intlon on the Progress of Mathematical and Physical 

 SdSBC*;' -uppicment to tnc)clopa?dla Britannic*,' vol. iv., 11. }. note: 

 ' incy. Brit.,' hth edit., TO). I., p. 6M, note. 



J How Impoitam to the progms of the science of phy.ical forces thl 

 calling the attention of philosopher, to Newton's views ban really been, will 

 appear from tlic foilowinit extract (mm Ih'- Theory of the Force of Gravity ' 

 by rrofww (bail!., published In the Philosophical Magazine' for Ueceinb.'r, 

 lt, p. . " Tb onto in dittani I,., been to long and to extensively re- 

 garded .. an ultimate principle, and not a. a temporary hypothesis admitting 

 errntualiy of explanation, that It require* aome degree of moral courage to 

 nulntatn a different theory. Science, In my opinion, la much indebted to Fro- 

 fcwor Faraday for bating recently directed attention to the opposite views 

 entertained by .Newton on thin point, and lor giving cxj rewloti to analogous 

 idea* of hi own. (nee the Lecture on the Conaenration ol Force, in the ' I hil. 

 Ma*.' : . ' '1 he lectMi-irfi mil i,, .,.. 



Miary ; but Faraday, an v,e hate Ken nbove, had originally 



.NI ton's tl.wn in 1853 ; and he regarded tie 

 aa ao great, that be bad recalled them alao In his dlseouraes of the two f.nlowiru 



approximation to a perfect vacuum," ..:! the qu 

 stated, he says, apply to a barometric tube employed in an 

 he subsequently describe!. Observing, however, that the Torricellian 

 vacuum is unfortunately applicable to very few physical investigations, 

 as no instrument of any kind can be introduced into it, nor even any 

 substance which is acted upon by mercury, and noticing the imper- 

 fection of the ordinary air-pump and of M. Kegnault's method of 

 pushing the exhaustion further alter the valves have ceased to act, he 

 proceeds to describe, in the following terms, a process by whir 

 very little trouble, a vacuum may be obtained in the ordinary receiver 

 of an air-pump, so perfect that the residual air exerts no appreciable 

 elastic I 



" Into the receiver of an ordinary air-pump, which is not required to 

 exhaust further than to 0"J inch, or even 0'5 inch, but which must 

 retain the exhaustion perfectly for any length of time, two open 

 vessels are introduced, one of which may be conveniently placed above 

 the other; the lower vessel containing concentrated sulphuric acid, 

 the upper a thin layer of a solution of caustic potash, which has 

 been recently concentrated by ebullition. The precise quantities of 

 these liquids is not a matter of importance, provided they are so 

 adjusted that the acid is capable of desiccating completely the potash 

 solution, without becoming itself notably diminished in strength, but 

 at the same time does not expose so large a surface as to convert the 

 potash into a dry mass in less than five or six hours at the least. The 

 pump is in the first place worked till the air in the receiver has an 

 elastic force of U'3 or ii'-i inch, and the stop-cock below the plate is 

 then closed. A communication is now established between the tube 

 for admitting air below the v.dves, and a gasholder containing car- 

 li..ni.- acid, which boa been carefully prepared so as to exclude the 

 presence of atmospheric air. After all the air has been completely 

 removed from the connecting tubes by alternately exhaustin 

 admitting carbonic acid, the stop-cock below the plate is opened, and 

 the carbonic acid allowed to pass into the receiver. The exhaustion is 

 again quickly performed to about the extent of half an inch or lea*. 

 li a very perfect vacuum is desired, this operation may be again 

 Urenie accuracy is required, it may be (terfonued a 

 third time. It is not likely that anything would be gained by carry- 

 ing the process further. On leaving the apparatus to itself, t!, 

 bonic acid which hax displaced the i. idual air is absorbed by the 

 alkaline solution, and the aqueous vapour is afterwards removed by 

 the sulphuric acid. The vacuum thus obtained is so perfect, that even 

 after two operations it exercises no appreciable tension." 



Even after this limit has been reached, the exhaustion may be pushed 

 still further, " till it must become at last not less complete than the 

 Torricellian vacuum ; while at the same time by suppressing the 

 manometer, the existence of mercurial vapour may be altogether 

 prevented. The manipulation required to arrive at this result will 

 not interfere with the presence of even the most delicate instruments 

 in the receiver." In an experiment which Dr. Andrews describes, the 



theoretical residue of air would be j ' of the entire quantity in the 



receiver, which would cause a depression of j !0 3 of an inch only, and 

 this, he says, must have been nearly realised. Such a vacuum has 

 remained without the slightest change for fourteen days. ' Phil. Mag.,' 

 Feb , 1852 ; ' Quart. Journ. of Chem. Soc.,' vol. v., p. ISiM'.il!. 



Unless the still more perfect carbonic-acid vacua employed by 

 Mr. Uaesiot, and about to be described, should be obtainable in com- 

 paratively large vessels adapted for the observation of instruments, Ac., 

 towards which result, indeed, Mr. Uastdot has made some approach, 

 Dr. Andrews's method appears to be the moat eligible lor the 

 generality of exact researches for which a vacuum is required. 



In a paper 'OntlieKlci-tne.il 1'heiioinena exhibited in Vocuo,' by 

 Sir H. l>avy, in the 'Philosophical Transactions' for ],v 

 some experiments which he made for the purpose of elucidating ' the 

 u of electricity to space, as neatly void of matter as it can be 

 made on the surface of the earth." He repeated the electrical experi- 

 ments with the Torricellian vacuum of Morgan and Walsh, and 

 instituted others with similar vacua above a difficultly fusible amalgam 

 of mercury and tin, and above fused tin. With the results as bear- 

 ing on the theory of electricity we ore not at ] : but 

 these approximations to vacuous space were, of course, in i 

 .iiini'-pliercs of the vapours of the metals -h of 

 excessive rarity ; and a calculation made by Mr. ' 

 may be cited, as indicating how minute must have !>< -u tin- quantity 

 of matter which they contained how great must have been it 

 " attenuation," in the language of the present day which is a point of 

 Information important to the subject of this article. Considering the 

 clastic force of vapour of - " to be equal to raise by it* pressure 

 about '45 of an inch of mercury ; the relative strengths ot vapour will 

 be, reckoning the boiling points nil in m .vj , for mercury at 600", 



000016015, and for tin, at 0000, 37m;., preceded by 48 zeros. 



The data on the diminution of the density of vapour* by diminution 

 of temperature supplied in this case by the chemist to the mathema- 

 <y in come degree erroneous, and the results would 

 r -d by the limit ' '< ! r . vi i v -uUt.niec at a e 



tempeiatiire which Faraday, a lew years after, rendered so highly 

 probable, if not certain, but the latter would operate tour 

 density of the metallic vapours in question ; and we may, all things 



